I have now produced a few demos on the 718, data-limited version of POSEIDON. The general summary of the work is that:
Here I focus on the second scenario and in particular I run a management-strategy evaluation using the LBSPR policy suggested in Hordyk, Loneragan & Prince (2015) to change effort.
More precisely, we measure SPR for L. malabaricus and we reduce total allowable effort \(E_t\) whenever SPR is below 40% through the formula: \[ v_t = 0.3 \times \left( \frac{SPR_t}{40\%} - 1 \right)^3 + 0.05 \left( \frac{SPR_t}{40\%} - 1 \right) \] \[ E_{t+1} = E_t v_t \] Effort is never allowed to increase or decrease by more than 10% (i.e. \(v_t \in [0.9,1.1]\)) year on year.
Where \(SPR_{\text{Target}}\) is 40%. We compute SPR by analyzing the catches made by 18 local long-liners, 16 long-range long-liners and 4 gillnetters, as in the CODRS data-set until DEC 2019, and TNC formula (i.e. linear selectivity).
We perform the MSE over 181 separate parameterizations of the model, each representing an “accepted” fishery, roughly congruent with current landings and SPR trends.
For each we checked the long-term (30 years) effect of imposing the LBSPR based policy described above for different assumed values of \(\frac M K\) (which we assume will remain unknown).
Effort in POSEIDON translates to number of days the fishery is open (“365” initially).
Much like the single-species demo, the main take-aways are:
## [1] 25
## [1] 40
In practice, the TNC formula is slightly more pessimistic than the Hordyk one, i.e. it outputs slightly lower SPRs (at all \(\frac M K\) and regardless of whether gillnetters are added in the sample or not).
The main problem is that while the SPR itself is only a few percentage points lower, because it is consistently so over the years you end up with large changes in depletion and effort in the long run.
Overall this means that TNC formula is more risk averse: lower effort higher depletion in the long run.
In terms of landings the effect is mixed: lower landings if the \(\frac M K\) was low but higher landings otherwise (stocks recovered faster).